Most conventional idle speed control systems involve servo-control of engine airflow either with a movable throttle stop or with an air bypass valve. There is a relatively long delay between any change in airflow and the corresponding change in idle speed. Therefore, the controller must have a fairly high degree of damping to maintain stability, and the response of the system to changing load conditions is rather slow.
Particularly with low idle speeds, desirable for low fuel consumption, there is a danger of the engine stalling with the sudden application of a load or combination of loads, such as the air conditioning compressor, power steering or alternator.
In these systems, if an attempt is made to speed up response by reducing the damping, over-shooting and oscillations in engine speed may occur. These conditions are objectionable by themselves, and may also result in stalling.
In addition, if the airflow is increased too suddenly, delays in fuel response and intake manifold fuel accumulation effects may cause transient disturbances in the fuel air ratio which can aggrevate the stability problem.
By temporarily changing the spark advance in response to idle speed changes due to changing engine load, either by retarding or advancing the value, the idle speed can be quickly moved back to the targeted value and held there until the air bypass system has a chance to catch up. As the air bypass valve is properly positioned, the spark advance is slowly changed back to its steady state value. This strategy allows the automatic idle speed actuator valve or motor to operate in such a manner that the system is very stable without the undesirable effects of slow response.
Although particularly well suited for control in an air bypass system, such a method can also be used in conventional carbureted engines and in throttle body injected engines and in multipoint fuel injected engines, all with movable throttle stops to control the spark advance in the same manner as herein described.